Tire testing device

ABSTRACT

This invention relates to a tire testing device, and in particular to a device for testing the tires of aircraft. The invention provides a tire testing device including a pressure sensor and a temperature sensor, allowing the calculation of the effective pressure at a reference temperature; the tires can therefore be tested hot or cold. The device can also include an oxygen sensor, so that the oxygen content of the tire can also be determined.

FIELD OF THE INVENTION

[0001] This invention relates to a tire testing device, and inparticular to a device for testing the tires of aircraft.

BACKGROUND TO THE INVENTION

[0002] Aircraft tires are required to contain a maximum of 5% oxygen.The reason for this is that if tires become heated to more thanapproximately 200° C. (which may occur if a set of brakes is binding ordragging), the chlorobutyl in the tire material begins to break down andto produce isoprene, and isoprene and oxygen together form anauto-ignitable mixture. The auto-ignition of aircraft tires might havebeen the cause of an unknown number of previously unexplained losses ofaircraft.

[0003] The instance of such auto-ignition is likely to increase becauseit is becoming increasingly common to fit aircraft with carbon brakes.Carbon brakes are advantageous since they are lighter, more efficientand longer-lasting than conventional brakes. However, whilstconventional brakes melt at around 400° C., carbon brakes are effectiveup to around 1100° C., so that the temperature which can be generated ina binding brake, and therefore which can be transmitted to an aircrafttire, is correspondingly increased.

[0004] Because of the known problems with auto-ignition, there is amandated limit of 5% oxygen in aircraft tires; the presence of such lowconcentrations of oxygen prevents auto-ignition in the presence ofisoprene.

[0005] Aircraft owners and users seek to meet this limit by filling thetires with nitrogen. Aircraft tires may require a pressure of aroundthirty atmospheres, for example, and so the air which is present in thetire before inflation becomes diluted by around thirty times. The airwhich was present in the tire before inflation will typically beatmospheric air containing around 21% oxygen; diluting this thirty timeswith pure nitrogen will result in an oxygen content within the tire of0.7%, well within the mandated limit.

[0006] However, the nitrogen which is used to inflate (or reflate) thetire will seldom if ever be pure, and in certain cases might containseveral percent oxygen. It is necessary that the nitrogen supply containless than 4.3% oxygen (for a thirty atmosphere pressure tire) so thatthe 5% level can be met.

[0007] In many cases, the owners of aircraft will use liquid nitrogen toinflate and reflate the tires of their aircraft, and this supply can beclose to 100% pure in practice. However, liquid nitrogen is expensiveand other less diligent owners and users instead utilise pressurisednitrogen gas. Often the pressurised nitrogen gas is purchased primarilyby price, and the quality (i.e. the percentage of oxygen present in thegas) is not certified and may not be known.

[0008] Also, at some airports the ground staff are not qualified or arenot trained to appreciate the significance of the oxygen content of thetires, and it has been known that tires be filled from an oxygen supplywhen the nitrogen supply was not available!

[0009] In addition to the oxygen content of the gas within the aircrafttire being critical, the pressure of the gas within the tire is alsoimportant. Thus, a tire which is under-inflated does not offer the samelevel of grip as it is intended to provide, and under-inflated tires arebelieved to be particularly suspectible to aquaplaning or hydroplaningwhen there is standing water on the runway (where a layer of waterbecomes trapped between the tire and the surface of the runway, reducingthe level of grip therebetween). There have been a number of aircraftaccidents in which the aircraft has skidded off the runway, andaquaplaning is believed to be a likely cause of the aircraft's failureto stop.

DESCRIPTION OF THE PRIOR ART

[0010] Pressure testing devices for the tires of aircraft are wellknown. One form comprises a mechanical gauge similar to that firstinvented over one hundred years ago. More modern devices use anelectromechanical sensor.

[0011] However, with such devices it is only feasible to test thepressure of the tires when they are at a known reference temperature,and this typically means that the tire must be at or close to theambient temperature, otherwise the hot gas within the tire will be at agreater pressure than the corresponding gas when cold, and thetemperature-induced variation will render the pressure readingunreliable.

[0012] In the United States, for example, the Federal Aviation Authority(F.A.A.) has expressed the wish that tire pressures be tested every day,but the airlines have indicated that this cannot be achieved in practicebecause aircraft are often in continual operation for up to three weeksat a time, and the aircraft is not on the ground within this period forlong enough for the tires to cool sufficiently for reliable testing totake place.

[0013] Oxygen testing devices are also available, by which the oxygencontent of an aircraft tire can be tested. However, the use of thesedevices is not universal because of the time taken to undertake thetesting. Thus, it has been estimated that to test the pressure andoxygen content of every tire on a large aircraft can take up to twohours, and this is longer than the desired turn-around time for mostaircraft (regardless of the time necessary for the tires to coolsufficiently for a reliable pressure test to be carried out).

SUMMARY OF THE INVENTION

[0014] It is an aim of the present invention to provide a tire testingdevice which can be used when the aircraft tire is hot or cold.

[0015] It is another aim of the present invention to provide a tiretesting device which can test the pressure and oxygen content of thetire in a single operation, i.e. only a single application of the valvehead upon the tire valve needs to be undertaken.

[0016] According to the invention therefore, there is provided a tiretesting device including a pressure sensor, characterised in that thedevice also includes a temperature sensor.

[0017] Preferably, the device has means to store a record of the volumeof the tire, and means to calculate an effective pressure at a referencetemperature. In the preferred embodiments the effective pressure at areference temperature can be calculated by the device, but in other(less preferred) embodiments the pressure calculation can be carried outseparately, e.g. by a computer or other device to which the measuredpressure and temperature are downloaded.

[0018] It is known that the relationship between the pressure,temperature and volume of a given quantity of gas are related to eachother (by Boyle's Law), and knowledge of the volume of gas within thetire can enable a pressure reading at any particular temperature to beconverted to a pressure reading at another (reference) temperature.Accordingly, if it is determined that the pressure of the tires shouldbe measured at 0° C., and the actual pressure is measured at 50° C.,then the equivalent or effective pressure at 0° C. can readily becalculated.

[0019] The ability of the device to measure the tires at any giventemperature, and in the preferred embodiments to calculate the pressureat a reference temperature, enables the device better to check anyleakage of gas which is occuring from a tire. Thus, with conventionalpressure test apparatus it would still not be possible to obtain anyreally useful data even if an aircraft's tires were allowed to cool andbe measured every day, if the same aircraft is present in a hotatmosphere such as Arizona on one day, and a cold climate such as Alaskaon the following day, since the ambient temperature, and thus thetemperature of the “cold” tire, might differ by 40° C. between the twolocations.

[0020] Preferably, the device comprises a base unit and a valve head,the valve head being connected to the base unit by a flexible tube, thevalve head being adapted to connect to the tire valve and to allow theescape of a small amount of gas therefrom. Preferably also thetemperature sensor is located within the valve head, so that thetemperature of the gas is measured as close to the tire as possible, andthe likelihood of miscalculations occuring, because for example the gashas cooled on leaving the tire, are much reduced.

[0021] Desirably, the temperature sensor is a thermocouple. Desirablyalso, the pressure sensor is an absolute pressure sensor, i.e. apressure sensor which can compare a pressure to be measured againstvacuum. Usefully, the pressure sensor is a pressure transducerconfigured as a strain gauge, such as that produced by Kistler ofSwitzerland under model number MER 180.A.20. The “20” element indicatesthat the sensor can operate over a range of 20 bar which is believed tobe sufficient for the majority of applications, but other sensors can beused, providing a greater (or lesser) range, if desired.

[0022] Desirably, the device also includes an oxygen sensor.Accordingly, the device can measure the pressure of the gas in the tireand can also measure the oxygen content at substantially the sametime—in particular requiring the operator to extract only a single“shot” of gas from the tire for both the pressure and oxygen tests. Inthis way, if the pressure of all of the tires is measured every day, arecord of the oxygen content can also be taken every day, and thereflation of the tire with poor quality nitrogen (i.e. containing alarge percentage of oxygen), or even with oxygen instead of nitrogen,can quickly be ascertained.

[0023] Preferably, the oxygen sensor is located in the base unit. Thus,it is not necessary that the oxygen content be measured close to thevalve since this content will not vary between the valve and the baseunit. Preferably also, the base unit contains a chamber into which thetire gas can be passed, the oxygen sensor being located within thechamber. Desirably, the device includes means to purge the chamber, sothat the gas from a previous tire measurement does not contaminate thetest for the oxygen content of the next tire.

[0024] Usefully, the base unit is portable, and is ideally designed tobe hand-held so that it may be carried around the aircraft, andspecifically carried to each tire of the aircraft which is being tested.Preferably, the base unit includes a clip so that it may be suspendedfrom a belt worn by the operator.

[0025] Accordingly, the device will be portable and the base unit willpreferably contain at least one battery to provide electrical power. Thebattery is preferably rechargeable, but the device also ideally containsa non-rechargeable battery bay, so that non-rechargeable batteries canbe purchased and used in the event that the rechargeable battery has notbeen sufficiently recharged prior to use.

[0026] Desirably, the base unit includes interface means by which thedata stored therein can be downloaded to a computer such as a mainframecomputer. The device can have a volatile memory in which can be storedthe data corresponding to the oxygen content, the pressure measurementand the calculated effective pressure, of the tires of each aircraftwhich the operator has tested, and a permanent record of thosemeasurements can be kept within the computer or outputted therefrom inpermanent form.

[0027] Usefully, the device takes periodic measurements of the ambientatmosheric pressure and temperature, and can utilise those measurementsin converting the measured pressure of the tire gas to the effective(reference) pressure. Thus, it is necessary also to account for theatmospheric pressure when calculating the effective pressure, and thedevice can cater for changes in the atmospheric pressure when making theeffective pressure calculations. Accordingly, the pressure can becalculated at a reference temperature and also at a referenceatmospheric pressure, so that the pressure within the tires of the sameaircraft can be reliably measured at both sea level (e.g. Miami) andaltitude (e.g. Denver), without the relative change in atmosphericpressure between those locations affecting the reliability of theeffective pressure reading.

[0028] Preferably, the device has a “stand-by” mode in which the displaymay be switched off during periods of inactivity. However, the ambientpressure and temperature readings can continue to be taken periodicallyduring these stand-by periods. The device can return to full operationalmode when the operator activates a switch, though preferably this isautomatic, and for example occurs when a significant rise in pressure ortemperature is detected, indicating that a measurement is to take place.

[0029] Since the device will need to include a record of the volume ofthe tire, it has a permanent memory containing a database of the volumeof the tires of the particular user's aircraft. Thus, the volume of gaswithin all available aircraft tires can be calculated or referred to,and the permanent memory of the device can be used to store the volumeof the tires of each of the user's aircraft. In addition, the devicewill preferably include a display means by which the operator will bedirected sequentially to each tire of the aircraft, and the device willautomatically calculate the effective pressure by referring to thevolume of each particular tire stored within its permanent memory.

[0030] The device will preferably have a control button which permitsthe operator to select the aircraft which is to be tested; the selectionmay be made by aircraft type, but preferably is made by reference to theaircraft's unique alphanumeric code, so that a record of the pressureand oxygen content of each tire of each aircraft can be maintained.

[0031] Desirably, the device has a display screen. Desirably also thecontrol means of the device can cause a representation of the aircraftto appear on the display screen, the control means also identifying eachtire of the aircraft upon the display screen in the sequential order inwhich the tires should be tested.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The invention will now be described, by way of example, withreference to the accompanying schematic drawings, in which:

[0033]FIG. 1 shows the valve head of the device;

[0034]FIG. 2 shows the base unit of a first embodiment of tire testingdevice according to the invention;

[0035]FIG. 3 shows one possible output of the display screen of thedevice; and

[0036]FIG. 4 shows the base unit of a second embodiment of tire testingdevice according to the invention.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0037] The valve head 10 shown in FIG. 1 is substantially ofconventional design, and includes an opening 12 into which a part of thetire valve 14 of an aircraft tire (not shown) can be fitted. In knownfashion, the opening carries a seal 16 against which the valve can besealingly engaged, and means (not shown) to open the valve and allow airto escape from the tire and into the body 20 of the valve head.

[0038] Within the body 20 of the valve head, and immediately adjacentthe opening 12, is located a temperature sensor 22. In this embodimentthe temperature sensor 22 is a thermocouple, specifically a type Tthermocouple having a positive element of copper and a negative elementof constantin (a copper nickel alloy). The effective temperature rangeof such a thermocouple is approximately −185° C. to +300° C.

[0039] The output of the thermocouple 22 is an electrical signalindicative of the temperature of the gas escaping from the tire. Theelectrical signal is carried by electrical leads 24, which pass alongthe hollow interior of the body 20 and also along the hollow interior ofthe flexible tube 26 which connects the valve head 10 with the base unit30 (FIG. 2) or 130 (FIG. 4). As can be seen in FIG. 2, the hollowinterior of the flexible tube 26 opens into a cavity 32 within the baseunit, and the electrical leads 24 pass from that cavity, through apressure seal 34, and terminate upon the printed circuit board 36.

[0040] The cavity 32 communicates with a further cavity 40 by way ofconduit 42, and further cavity 40 communicates with a chamber 44 by wayof conduit 46. In this way, gas which is extracted from the tire is fedalong the flexible tube 26 and enters the cavities 32 and 40 and thechamber 44.

[0041] Adjacent to cavity 40 is located a pressure sensor 50, in thisembodiment a solid state pressure sensor manufactured by Kistler ofSwitzerland, and specifically model number MER18A.20. The pressuresensor is located within a recess 52 in the body of the base unit 30,which recess carries an annular seal 54 to prevent the escape of any ofthe gas. The pressure sensor is retained within the recess 52, and asealing pressure retained upon the annular seal 54, by a locking ring 56which is suitably threaded and locates within a correspondingly threadedenlarged section of the recess 52.

[0042] It will be understood by those skilled in this art that thispressure sensor is an absolute pressure sensor, having a vacuum to oneside of its working element. The pressure sensor functions by measuringthe strain on the working element, which strain is indicative of thepressure differential between the two sides of the element. Otherpressure sensors could be used, but it is expected that a strain gaugetype of sensor such as that described would be most suitable.

[0043] The output of the pressure sensor 50 is an electrical signalindicative of the pressure within the cavity 40; electrical leads 60communicate the output of the pressure sensor 50 to the printed circuitboard 36.

[0044] The chamber 44 contains an electro-chemical oxygen sensorrepresented by panel 62. Commercially available oxygen sensors of theelectrochemical type are known, and are available from City TechnologiesLimited of Portsmouth, U.K.—a suitable sensor is that sold by thiscompany under reference number C/YO₂. In alternative embodiments,however, another type of oxygen sensor could be used, such as a solidstate sensor similar to that disclosed in WO94/23289 but suitablyconfigured to detect oxygen. A suitable solid state oxygen sensor isavailable from Omega High Technology Sensors Limited, Unit 8, AstonFields Trading Estate, Sugarbrook Road, Bromsgrove, Worcestershire, B603DW, England. A solid state oxygen sensor is likely to be preferred bymany operators.

[0045] The output of the oxygen sensor 62 is an electrical signalcorresponding to the proportion of oxygen present in the gas within thechamber 44, and that electrical signal is communicated to the printedcircuit board 36 by the electrical leads 64.

[0046] The device also includes a display screen 70. Suitably thedisplay screen is a liquid crystal display. Since liquid crystal displayscreens are highly susceptible to irreversible temperature damage thescreen is separated from the remainder of the base unit by an air gap72, and engages the remainder of the base unit by way of thermallynonconductive (or poorly conductive) mounts 74. In this way, the screen70 can be protected from the high temperatures which might occur withinthe remainder of the base unit because of the hot tire gases and/orbecause of incident solar radiation in a particularly hot climate.Preferably, the mounts 74 are resilient and flexible so that the screen70 is also protected from physical shocks to the base unit 30.

[0047] One possible output of the display screen is shown in FIG. 3.Thus, when the device is first switched on the operator is asked toselect the aircraft which is to be tested. Preferably, the permanentmemory of the device includes only those aircraft within the fleet ofthe particular aircraft owner or user, so that the data for aircraftwhich are not in the fleet do not need to be retained in the memory. Thebase unit 30 (and also the base unit 130 of FIG. 4) will have a controlbutton (not shown) which the operator can use to select the aircraft.Usefully, the control button is a rocker switch by which the operatorcan move a cursor up or down through a menu of the fleet's aircraftshown on the display means. Alternatively, a control button can bedepressed repeatedly to scroll through a menu of the fleet. The displaymeans will preferably show the aircraft type (e.g. “DC-10”) and theaircraft's unique alphanumeric code.

[0048] Once the aircraft to be tested has been selected, the displaywill reveal a representation 80 of the aircraft. The representationshows each wheel or tire, and an arrow 82 is highlighted showing theoperator which tire to check, in this case one of the nose-wheel tires84. The microprocessor accesses the memory for the volume of thatparticular tire, so that the subsequent pressure measurement andtemperature measurement (together with the ambient pressure andtemperature measurements which have previously been (recently) recorded)can be used to convert the measured pressure into an effective pressure.

[0049] Alternatively, the arrow 82 can be omitted and instead therepresentation of the tire to be tested can flash intermittently uponthe display panel 70, with tires which have already been tested beingshown as solid and tires which have not been tested being shown inoutline.

[0050] The measured pressure and the calculated effective pressure canbe shown on the display panel 70, to confirm to the operator that thecheck has been completed successfully. In addition, the result of theoxygen content test can be displayed as a pass/fail, e.g. the letters“OK” can be displayed, or the letters “OK” ruled through with a crossfor example (which it is believed will be understood by nationals ofmost countries of the World without requiring a translation)—if theoxygen test fails the operator can press a control button to display theactual oxygen content measured as a percentage.

[0051] Notwithstanding that the display means will preferably(initially) show only a pass/fail condition for the oxygen content, themicroprocessor stores the actual percentage, and this actual percentagecan be downloaded to the computer, so that changes in the actualpercentage of oxygen within the tire can be checked.

[0052] Following the testing of the first tire, the arrow 82 (or otherindicator) on the display 70 will move to another tire, and the operatorcan proceed to test that tire.

[0053]FIG. 4 shows a second embodiment of base unit 130. In thisembodiment, the temperature reading, which is communicated from athermocouple (not shown in FIG. 4) along electrical leads 24 (only oneof which is shown in FIG. 4), is communicated to a first printed circuitboard 86 which carries a first microprocessor 88. The pressure sensorwhich is located in cavity 40 (and which is not shown in FIG. 4, but maybe identical to that of FIG. 2) also communicates with the first printedcircuit board 86, and in turn with the first microprocessor 88, by wayof electrical leads 60 (only one of which is shown in FIG. 4). Theoxygen sensor which is located in chamber 44 (and which again is notshown in FIG. 4, but may be identical to that of FIG. 2) alsocommunicates with the first printed circuit board 86, and in turn withthe first microprocessor 88, by way of electrical leads 64 (only one ofwhich is shown in FIG. 4).

[0054] The first printed circuit board 86 is shown schematically only,and in practice would carry components in addition to the microprocessor88, which components could if desired condition or modify the signalsreceived along electrical leads 24, 60 and 64 before transmission to themicroprocessor 88.

[0055] The cavity 40, chamber 44, and first printed circuit board 86 arelocated in first housing part 90, which housing part also carries afirst connector 92 which receives signals along lead 94 from the firstmicroprocessor 88 (by way of other components to condition or modify thesignal(s) if desired). Only one electrical lead 94 is shown between theprinted circuit board 86 and the first connector 92, but in practicemany electrical leads could be provided depending upon the number ofseparate signals required to be communicated.

[0056] The base unit 130 also includes a second housing part 96 which isdesigned to interconnect with, and be secured to, the first housing part90 by way of suitable securing means (not shown). When secured togetherthe first and second housing parts 90,96 comprise the base unit 130,which together with a flexible tube and valve head such as that shown inFIG. 2 comprise a complete tire testing device according to theinvention. Also, when secured together the first connector 92 of thefirst housing part 90 is connected to a second connector 98 carried bythe second housing part 96. The second connector 98 communicates with asecond microprocessor 100 carried upon a second printed circuit board102 within the second housing part 96. In this way, signals can becommunicated between the first and second microprocessors 88,100 (andvice versa) by way of the first and second connectors 92,98.

[0057] The second microprocessor 100 drives the outputs from the tiretesting device, i.e. the display screen 70 and also the data outputs,i.e. the data to be downloaded to a computer, for example. Once again,the second printed circuit board 102 would typically carry othercomponents to modify or condition the input and/or output signals asdesired.

[0058] The separable first and second connectors 92,98 between the firstmicroprocessor 88 and the second microprocessor 100 are provided topermit the first housing part 90, including all of the sensors and thefirst microprocessor 88, to be removed from the device for calibration.Thus, notwithstanding that it might in some embodiments be possible tocalibrate the device by way of a computer, or perhaps remotely by way ofthe internet, it is envisaged that some customers would prefer not tohave to carry out the routine calibration work. Such customers couldtherefore periodically (perhaps once each year) send the first housingpart 90 and its componentry to the supplier (or to an approvedcalibrator) for calibration, whilst a replacement (pre-calibrated) firsthousing part could be provided to the customer so that the device cancontinue to be used. Since recalibration can be made verystraightforward for the customers, it is expected that routinerecalibration would be a desirable, or perhaps mandatory, feature ofoperation of the device to ensure that the sensors and the firstmicroprocessor retain their full operational effectiveness and accuracy.

[0059] The first and second microprocessors 88,100 can each have asecurity coding, so that a particular first housing part 90 (and itssensors and first microprocessor 88) can only be used with a particularsecond housing part 96 (and its display device 70 and secondmicroprocessor 100). Such an arrangement would prevent the operatorbeing able to swap around the respective housing parts from more thanone device, which might allow the avoidance or delay of routinerecalibration.

[0060] In addition, it is arranged that the first microprocessor 88, oranother component carried by the first printed circuit board 86,includes an analogue to digital converter, so that the signals beingcommunicated to the second microprocessor 100 are digital.

[0061] The second housing part 96 has four terminals 104 shown connectedto the second printed circuit board 102. These terminals 102 areadditional to the RS 232 data communication terminal (not shown) whichis expected to be a desirable feature of the device. The terminals 104are adapted for connection to corresponding terminals on the batterycharger (not shown) to which the device will be periodically connected,usefully at the end of each work shift. The terminals 104 can be fittedwith suitable contact surfaces, as will be well-understood by thoseskilled in the art.

[0062] Two of the terminals 104 are to provide electrical power torecharge the batteries (not shown), which are carried by the secondhousing part 96, whilst the other two terminals 104 can communicate byway of the battery charger with a computer. The latter two terminals 104therefore correspond to the data input and data output terminals, bywhich the device can communicate with a host computer. It is expectedthat such an arrangement will be preferred over using the RS 232terminal for such communication, since the RS 232 terminal is likely tobecome damaged by repeated (perhaps daily) connections. Thus, it ispreferred to retain the RS 232 terminal for less frequent connection, asmight be required for remote calibration, diagnostic checks orengineering inspection, for example. Thus, it is envisaged that remotediagnostic checks can be carried out upon the device, and certain(software related) failures of the device can be detected and rectifiedremotely, perhaps by way of the internet or other remote communicationslink. Also, if additional aircraft are added to the fleet then the datafor the representation 80 of that aircraft, and the volume of each ofthe aircraft's tires, can be transmitted to the device (by way of the RS232 connection or by way of the data terminals 104, as preferred) andadded to its permanent memory, without the device having to be returnedto the manufacturer or supplier.

[0063] It will be understood that the battery charging terminals can beconnected to the batteries by way of suitable components on the printedcircuit board 102, which components can condition or modify the incomingvoltage and current if required. Preferably, however, the incomingvoltage and current require no such conditioning or modification, andthe battery charger supplies the desired recharging voltage and current.

[0064] As indicated, the device is likely to include a minimum number ofcontrol buttons, for example three control buttons. If it is desiredthat the device be PIN (personal identification number) controlled, sothat it may be used only by known operators, then the battery chargercan be fitted with a key-pad, suitably a numeric but perhaps analphanumeric key-pad, and the PIN entry can be made by way of thebattery charger and the data terminals 104. Using the battery charger inthis way helps to ensure that the operator will enter the PIN in acontrolled environment such as an office, and does not need to enter thePIN in an adverse environment such as outdoors in bad weather.

[0065] When the device is in use, it is necessary to ensure that theoxygen content of a previously-tested tire will not contaminate the gasof the subsequently-tested tire, and so the chamber 44 must be purgedbefore each test. In the first embodiment of FIG. 2 anelectrically-controlled fan 66 and a suitable valve (not shown) areprovided for this purpose. When the valve is shut the chamber 44 isclosed, and when gas is allowed out of the aircraft tire the pressurewithin the hollow interior of the flexible tube 26, the cavities 32 and40, and the chamber 44 rapidly become equal to the pressure within theaircraft tire. However, when the test has been completed the valve canbe opened and the fan 66 operated so as to expel the gas from within thechamber 44 (and also from within the cavities and conduits of thedevice) and replace this with atmospheric air.

[0066] Alternatively (and preferably) the chamber 44 can be purged withgas from the next tire to be tested. In such embodiments the component166 (FIG. 4) is a valve, preferably a Schrader valve. The Schrader valvecan be opened by an electric motor (not shown). It is necessary that thevalve 166 remains open for a sufficient time to permit the gas withinthe chamber 44 to be flushed out by gas arriving along the flexible tube26 (FIG. 2). The valve 166 is subsequently closed, allowing the chamber44 to fill with gas from the tire being tested. It is necessary to purgeall of the gas remaining from the previous test which is present withinthe chamber 44 (and the remainder of the device) before the oxygencontent of the tire being tested can be accurately determined. Thevolume of the tube 26, cavities 32, 40, chamber 44, and conduits 42,46of the device should be minimised as far as is practicable, and in aworking design this volume is expected to be about 30 cc. To ensurecomplete purging with such a design it can be arranged that around 100cc of gas is purged, i.e. 100 cc of gas is allowed to escape through thetire valve and pass into (and through) the device.

[0067] To avoid faulty or unreliable pressure readings, as might occurif the instrument is not applied correctly to the tire valve, thepressure sensor 50 takes sequential readings and only accepts a readingif three sequential readings are within a predetermined relative range.Experiments have shown that when using a strain gauge pressure sensor aspreferred, the sensor will give a reading of approximately 98% of theactual pressure after approximately thirty seconds, and will give a 100%reading after approximately ninety seconds. It is believed that ninetyseconds is too long to expect an operator to hold the valve head 10 uponthe tire valve of the tire being tested, and so in practice it isexpected that the actual pressure reading will be extrapolated from the98% reading.

[0068] The microprocessor 88 has a permanent and a volatile memory(RAM). The permanent memory will contain the volume of each tire whichwill be tested by the device, and that information can be used by themicroprocessor to convert the measured pressure at the measuredtemperature into an effective pressure measurement at a predeterminedreference temperature. When the microprocessor 88 has calculated theeffective pressure at the reference temperature and the referenceambient pressure, this is stored within the volatile memory of thedevice. The measured oxygen content of the tire is also stored in thevolatile memory.

[0069] When the pressure and oxygen content of all of the tires of theaircraft have been measured, the results can be downloaded from thevolatile memory of the device to a computer or other more permanent datarecorder. Alternatively, the downloading of the data may occur once eachshift, suitably at the end of the operator's work day, when the datafrom several aircraft can be downloaded together.

[0070] One suitable sequence of operations of the device utilising abase unit 130 of FIG. 4 is as follows. Firstly, the valve head 10 isapplied to the valve of a tire. The pressure rise within the cavity 40(which is due to the passage of gas along the flexible tube 26) isdetected by the pressure sensor 50 which activates the firstmicroprocessor 88. The valve head 10 is retained upon the valve for aperiod of 30 seconds, after which the microprocessor 88 interrogates thepressure readings from the pressure sensor. Sequential pressure readingsare taken (approximately 1 microsecond apart), and if three sequentialreadings are sufficiently close together (preferably within 3.5% of eachother) the average of those readings is determined to be 98% of theactual pressure. If, however, the sequential readings are notsufficiently close together (e.g. they differ by more than 3.5%) it isassumed that the operator has moved the valve head out of adequatecontact with the valve, and more pressure readings are taken until threesequential readings are sufficiently similar. When three sufficientlysimilar sequential pressure readings have been obtained the firstmicroprocessor 88 extrapolates the pressure reading to arrive at theactual (100%) pressure value.

[0071] After completion of the pressure sensing, the process continueswith the oxygen test and temperature reading. Specifically, the firstmicroprocessor 88 instructs the motor to open the valve 166 and purgethe gas from within the chamber 44 and remainder of the device. To dothis the motor drives a rod which in turn engages the pintle of theSchrader valve 166 (none of which is shown in the drawings, but whichwill be readily apparent to a skilled worker). The position of the rodis determined by a rotary encoder mounted on the motor, and in order toensure that the rod opens the valve for the predetermined period of timethe rod is first withdrawn to its fully retracted position (whereuponthe current rises towards the stall level and is cut off before themotor stalls—this is to prevent the motor jamming), and then drivenforward a predetermined distance necessary for the rod to engage thepintle. The rod is moved further so as to open the pintle and allow theescape of gas, and then closed after a predetermined time. The rod ismoved away from the pintle initially so that its position can beaccurately and reliably ascertained, i.e. it is not assumed that theposition of the rod will be known sufficiently accurately unless it isfirst reversed to a known starting position.

[0072] As above indicated, whilst the “dwell volume” of the device islikely to be around 30 cc it is desired to purge around 100 cc of gas.The time for which the valve 166 must be open to purge 100 cc of gas atthe pressures involved is likely to be very short, and the valve 166 mayinclude a restriction to limit the rate of outflow of gas so that thedesired purge quantity can be effectively controlled.

[0073] It is also desired that the temperature measurement be taken bythe thermocouple 22 only after the device has been purged. The reasonfor this is that only a small volume of gas will flow through the valvehead when it is first connected to the valve, i.e. whilst the valve 166remains closed, and the thermal mass of the thermocouple itself willtend to corrupt any temperature measurements taken at that time. Afterthe period of thirty seconds has passed, after which the pressurereadings are taken, the temperature of the valve head will approach thatof the gas expelled from the tire. Also, the additional 100 cc (orthereabouts) of gas which flows through the valve head during thepurging process will result in a far more reliable temperaturemeasurement, i.e. the temperature measured by the thermocouple 22 willbe much closer to the actual temperature of the gas. Even so, however,it is expected that the measured temperature will be slightly below theactual temperature, and the deviation can be tested empirically, and thefirst microprocessor 88 include an algorithm to correct for this.

[0074] Preferably, the device also contains an electronic clock whichcan record the date and time of each test, i.e. to record the date andtime on each occasion upon which the pressure sensor indicates that atire pressure test has been made. This data can be downloaded togetherwith the tire pressure and oxygen content. With such date and timerecordal, the authorities can check that the testing is being carriedout with sufficient frequency for every aircraft in a user or owner'sfleet. Also, the aircraft user or owner can readily check any gradualdecrease in tire pressure which may be occuring on a particular tire,and can perhaps use this information to determine which tiremanufacturer produces the tires which require the most and/or the leastreflation.

[0075] It is an additional advantage of the ability of the device torecord the time and date of each measurement that the date of the lastmeasurement can be ascertained. Thus, if the device ceases to work thedate and time of the last measurement (before it ceased to work) can bedetermined, and the operator who was perhaps responsible for thedevice's failure can be identified. Operator's are likely to take morecare of a device if it can be determined that their mis-use caused afailure of the device. Thus, notwithstanding that the device willpreferably be manufactured to withstand a two-metre drop test, it willnevertheless still be possible to cause the device to fail throughmis-use.

[0076] It will be understood that the relationship between the pressurewithin the tire, the temperature of the gas therein, and the volume ofthe gas, might not always be uniform, since the elasticity of the tire(and hence its volume) might change with temperature. However, suchchanges in elasticity are expected to have a minor effect so that therelationship is substantially uniform. If, however, experiment showsthat the relationship is not sufficiently uniform for some aircrafttires, the lack of uniformity can be determined empirically, and thefirst microprocessor 88 (or the single microprocessor of the embodimentof FIG. 2) can include an algorithm to correct for the lack ofuniformity.

[0077] In the drawings, the printed circuit boards 86,102, themicroprocessors 88,100 and the various electrical leads, are shownwithin the respective housing parts which are drawn as solid. However,it will be understood that in practice these components are located inchambers or cavities within the housing parts.

[0078] Though not shown in the drawings, the display screen 70 willpreferably have illuminating means by which the screen can beilluminated when the ambient light is insufficient to permit the displayto be clearly seen. Preferably the device can have a light sensor todetect the level of ambient light, so that the illuminating meansoperates automatically. In addition, the device can carry a furtherilluminating means adapted to shine light onto the tire valve so thatthe operator can more readily locate the valve and correctly place thevalve head 10 thereon.

[0079] Since the device will ideally be used in many countries of theWorld, it is preferable that the display screen 70 and the controlbuttons (not shown) use symbols or graphic representations rather thanwords, so as to avoid the requirement for translations. In addition, thedevice, and in particular the valve head and the control buttons, willtypically be large enough to be operated by a gloved hand, since gloveswill often be required in very cold and very hot climates. Thus, it isintended that the base unit be of a size and weight that it can readilybe held in one hand, so that the operator can hold the base unit withone hand and the valve head with the other hand. It is also intendedthat the control buttons be accessible to the operator whilst the baseunit is being held, also with one hand, so that the operator can accessthe controls buttons without needing firstly to remove the valve headfrom the tire valve.

[0080] The rechargeable battery (not shown) is designed to providesufficient electrical power for the device to be operated for aroundfourteen hours, which is longer than a work day for most operators. Inthis way, the device can be used for a whole work day before needing tobe recharged (in time for the next work day). Ideally, the volatilememory of the device is large enough to store data from many aircraft,and in particular more aircraft than can typically be tested in afourteen hour period. Thus, the pressure and oxygen data need only bedownloaded at the end of the operator's work day.

[0081] The base unit 30,130 should be designed to be resistant to damagecaused by sunlight, ozone, and skydrol, the latter being a lubricantused for aircraft.

[0082] Notwithstanding that in both of the embodiments show (FIG. 2 andFIG. 4) an oxygen sensor is included, it is possible to provide a tiretesting device able to test the pressure of the gas within the tires butnot the oxygen content. Such embodiments would require only the pressuresensor and the temperature sensor.

1. A tire testing device including a pressure sensor, characterised inthat the device also includes a temperature sensor.
 2. A tire testingdevice according to claim 1 in which the device has means to store arecord of the volume of a tire, and means to calculate an effectivepressure at a reference temperature.
 3. A tire testing device accordingto claim 1 which comprises a base unit and a valve head, the valve headbeing connected to the base unit by way of a flexible tube, the valvehead being adapted to connect to the tire valve and to allow the escapeof gas therefrom.
 4. A tire testing device according to claim 3 in whichthe temperature sensor is located within the valve head.
 5. A tiretesting device according to claim 3 in which the base unit comprises afirst part and a second part, the first and second parts beingseparable, the flexible tube being connected to the first part, thefirst part containing the pressure sensor and a first microprocessor,the second part containing a second microprocessor, interface means anda display means, the first and second parts having respective connectingmeans by which the first and second microprocessors canintercommunicate.
 6. A tire testing device according to claim 1 in whichthe device also includes an oxygen sensor.
 7. A tire testing deviceaccording to claim 6 having a base unit containing a chamber into whichtire gas can pass, the oxygen sensor being located within the chamber.8. A tire testing device according to claim 7 including means to controlthe escape of gas from the chamber.
 9. A tire testing device accordingto claim 1 including at least one battery to provide electrical power.10. A tire testing device according to claim 1 including interface meansby which data stored within the device can be downloaded to a computer.11. A tire testing device according to claim 1 having control means anda display means, the control means being adapted to display arepresentation of an aircraft on the display means, the control meansalso being adapted to identify each tire of the aircraft upon thedisplay means in a sequential order.